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I'm working on a design for a home-brew 10 metre transmitter for CW. My oscillator is running cleanly, and I need to add a (small, probably 1/2 ~ 1 Watt) power stage to it, and I need to "modulate" the signal--i.e. turn the output on and off :) I want this to be simple, but I also want to avoid a bunch of splatter during keying. I'm sure this must have one or more very standard approaches by now, but I've not found much information on the topic. In particular, I want to find a reasonable compromise between simplicity and clean signal. I would therefore appreciate guidance.

My thoughts so far are:

  • (EDIT) I think my simplest "good" route might be to put what amounts to a variable voltage divider (using a FET) between the oscillator output and the PA input. I figure I might use the key into the FET's gate, with some capacitance to soften the transition from on to off. Something like the sketch below. I've given no thought yet to the time constants on the switching of the FET, but that part I know I can handle :) However, I'm not sure how well this might work. I think I need both decoupling capacitors in the feed path between the oscillator and the input to the PA stage, but I suspect I'll need to provide some bias for the part of the path where the FET sits, otherwise, after dragging the level down to ground, it'll have no reason to return to a level that's above the gate voltage, and ... well, I'm pretty weak with FETs, but I don't think that's good. There might well be other issues here--please speak up and feel free to ridicule this if you have comments!

schematic

simulate this circuit – Schematic created using CircuitLab

  • build a single-transistor amplifier stage, and simply use the key to connect the oscillator output to the amp input. My concerns here are: 1) that the switch on/off, if it happens in mid-cycle (which seems virtually guaranteed) will create a very sharp rise or fall, and that will create splatter. 2) If the input impedance isn't high enough, might I pull the oscillator off frequency? (It's crystal controlled, but still).

  • as above, but with an emitter follower buffering to keep any "pulling" to a minimum. This obviously doesn't address the mid-cycle switching, and indeed might make it worse.

  • make the amp a regular common emitter, but with a FET in the emitter line, acting as a switch to introduce the lower of two parallel resistors defining the gain. I think this way I can effectively turn the gain of the output stage down to "very low" (although not completely off). I think I could also introduce a time constant into the on/off switching of the gate to "damp" the transition between output and no-output.

  • use a dual gate FET type device. This seems to have two problems. 1) I've never used one before, so there's far more unknown quantity involved, and 2) The options for such devices I've managed to locate so far are both limited and all SMD (I'd rather use old-world through-hole if possible!)

What would you suggest? One of these options, or something else entirely?

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  • $\begingroup$ This is a great question! We are glad to have you here on ham.SE. :-) $\endgroup$
    – Mike Waters
    yesterday
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Personally I wouldn't worry much about chirp (frequency pulling), unless it's really bad. It's not hard to copy, for a human anyway. It warms my heart whenever I work someone with a chirpy homebrew transmitter.

Key clicks, what you're calling splatter, is another matter entirely. You'll need some sort of circuit to shape your keying envelope or your key clicks will occupy an incredible bandwidth, possibly only limited to the bandwidth of your bandpass filter. 1 W is quite low power of course, but key clicks unfortunately are easily heard many kilohertz away (for egregious cases). And operators like to tune around to discover who has such an awful signal.

(Key clicks used to be a dirty Morse code contesting trick to keep one's frequency clear. No other station wants to be near in frequency to a loud clicky signal, because it's hard to copy when someone else's clicks are overlaid on the signal one is trying to copy. Certain well-known commercially-made transmitters were famous for making bad key clicks.)

Here's what a non-clicking signal looks like on an oscilloscope at audio frequencies (simulated, apparently).

An oscilloscope trace of a Morse code signal with no key clicks.  Two dits and a dah are shown, and the amplitude of all slowly grow and slowly decay.

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    $\begingroup$ Any thoughts or references on how to minimize the clicks? I think this was a key part of what I was concerned about. $\endgroup$ 2 days ago
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    $\begingroup$ Yup, this answer, while very helpful doesn't address the click part of the OQ. However, I agree with you about the chirps. :) I have seen some key click suppression circuits in some older east (ARRL) and west coast (E&E; later Bill Orr) handbooks. They are for tubes, but IIRC they should be relevant regardless. $\endgroup$
    – Mike Waters
    yesterday
  • $\begingroup$ At any rate, this answer shows what you should check for using an oscilloscope. It doesn't have to be an expensive one. $\endgroup$
    – Mike Waters
    yesterday
  • $\begingroup$ Indeed it does. I have a scope, so we're good there. $\endgroup$ yesterday

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